You surely know that ninjas rock, but ninja rocks are ceramic shards from broken spark plugs. Because they’re readily available, easy to carry and conceal, and break glass quietly (like a ninja!), ninja rocks have become a weapon of choice in “smash and grab” car burglaries.

The hard alumina fragments of broken spark plugs are great at breaking tempered glass (think car windows, but not windshields—those are laminated safety glass). The ninja rocks work better than chunks of brick—the usual readily available “resource” for the bad guys. The reason is that the small, sharp, and hard alumina fragments induce scratches that pierce through the residual stresses. Once the crack gets started, it propagates quickly, following the unavoidable rules of fracture mechanics (pdf). Watch the one-minute video above to see how ninja rocks compare to a brick when up against a car window.